Atomic Element #79

[TakeAStepBack]

WebElements Periodic Table of the Elements | Gold | Essential information
Brief description: most metals are metallic grey or silvery white whereas gold is characteristically a metallic yellow colour, in other words gold-coloured. Caesium is also gold coloured. The gold colour seems related to relativistic effects of the outermost gold orbitals.

Small amounts of other metals alloyed with gold change the colour as well as mechanical properties such as hardness. White gold for jewellery is formed by mixing palladium, silver, or nickel with gold, although the result is green gold with certain proportions of silver. White gold is commonly used for wedding rings in the USA. Addition of some copper gives "rose gold", a soft pink colour. Remarkably other colours such as purple (a gold:aluminium alloy), blue (a gold:indium alloy) and even black (a gold:cobalt alloy) may be formed.

Gold is usually alloyed in jewellery to give it more strength, and the term carat describes the amount of gold present (24 carats is pure gold). It is estimated that all the gold in the world, so far refined, could be placed in a single cube 60 ft. on a side. It is metallic, with a yellow colour when in a mass, but when finely divided it may be black, ruby, or purple.

It is the most malleable and ductile metal; 1 ounce (28 g) of gold can be beaten out to 300 square feet. It is a soft metal and is usually alloyed to give it more strength. It is a good conductor of heat and electricity, and is unaffected by air and most reagents.

Gold is readily available commercially and its price (see the UtiliseGold directory) changes day by day and is one of the most widely tracked commercial prices.

The most common gold compounds are auric chloride (AuCl3) and chlorauric acid (HAuCl4). A mixture of one part nitric acid with three of hydrochloric acid is called aqua regia (because it dissolved gold, the King of Metals). It is unaffected by air and most reagents. It is found free in nature and associated with quartz, pyrite and other minerals. Two thirds of the world's supply comes from South Africa, and 2/3 of USA production is from South Dakota and Nevada. Gold is found in sea water, but no effective economic process has been designed (yet) to extract it from this source

 

[TakeAStepBack]

Gold Assay

It is critical from the public's perspective that there is confidence in the claimed purity of any particular item made from precious metals. This confidence is provided by an 'assay' (test and assess) of the precious metal content of that item. It is impossible to tell the precious metal content of any item simply by looking at it. Precious metals such as old, silver, and platinum are too soft to use alone for making jewellery, cutlery and other goods. Quite properly they must be alloyed with base metals (which happen to be cheap) for manufacturing. The assay protects the consumer by ensuring sure that not too much base metal was used. It also safeguards responsible manufacturers by providing an independent assessment of quality and content that in which the public has confidence.

There has been an "Assay Office" at Sheffield in England since 1773 when local silversmiths won the right from Parliament to assay silver in Sheffield. The 1773 Act of Parliament appointed 30 local men as 'Guardians of the Standard of Wrought Plate in the Town of Sheffield' to supervise the work of the Office. In 1773 Sheffield already had an established tradition of fine silverware production and the number of Guardians who were also silversmiths was restricted to just ten to ensure that the Assay Office offered an independent and impartial service. This safeguard was to ensure the Office was run for the benefit of the consumer as well as the manufacturer. Once a piece was assayed, it was marked using a "hallmark", making the hallmark perhaps the oldest mark of consumer protection. The first UK Assay Office was and is based at Goldsmiths' Hall in London. It founded around 1300, and is from where the term "hallmarking" originates, meaning "marked in Goldsmiths' Hall".

While there are assay offices in the USA, there is no hallmarking scheme.

 

[TakeAStepBack]

Isolation
Isolation: it would not normally be necessary to make gold in the laboratory as it is readily available commercially. The most romantic way to extract gold is by panning it out from a stream in some pleasant valley but most such sources are now depleted. Panning relies upon the density of gold (which is very high) being much greater than that of the sand and other particulates. It therefore settles to the bottom of the pan. The amount of gold recoverable in this way is declining.

One suggestion regarding the golden fleece in the Jason and the Argonauts story (Greek mythology) is that the golden fleece is a consequence of gold mining. There are suggestions that, perhaps 1500 years ago sheep fleeces were stretched out over wooden frames and be submerged in streams. Gold particles swept down from from upstream deposits would then become embedded in them. The fleeces were then dried in trees before shaking or combing the gold out. Similarly, sheep fleeces may have been used on washing tables at alluvial gold mines with much the same effect. Perhaps such methods predated panning of gold from river sands.

Today, more often than not, gold is extracted from ores. These ores often contain relatively little gold. Some of these processes cause environmental concern. Much gold is recovered from ores that are low in gold concentration using a cyanide extraction process. Cyanide extraction was first used around 1887, when the MacArthur-Forrest Process was developed in Glasgow by John Stewart MacArthur. Many worry about the envoronmental effects of the cyanide extraction process and the risks of using cyanide on a large scale. There are three main steps.

The first step is leaching - the ore is crushed to a powder so as to expose the small gold particles. and mixed with water. The resulting mixture of powdered ore and water (the slury) is then reacted with cyanide in the presence of oxygen.

4Au(s) + 8CN-(aq) + 2H2O(l) + O2(g) → 4[Au(CN)2]-(aq) + 4OH-(aq)

The result is that electrons from oxygen are used to convert the gold metal into a Au(I) complex, [Au(CN)2]-. The acidity of the process must be slightly alkaline (pH 10.1 for instance) to minimise the release of highly toxic hydrogen cyanide while optimising the leaching rate.

The next stage is concentration. Once in solution the gold must be converted back to gold metal. One way to do this is by adsorption of the gold onto activated carbon. Most of the impurities are left behind in the solution. This would appear to cause some anion exchange of [Au(CN)2]- with anions associated with the carbon, the precipitation of insoluble AuCN, and the formation of some metallic gold within the carbon pore structure.

The final step is recovery and refining. Gold is stripped from the carbon by mixing it with NaCN and NaOH at 1108C forming a new solution of [Au(CN)2]-. This solution is now fairly pure as the activated carbon process removes many of the impurities. The gold is then converted back to elemental gold in the following electrolysis reactions known as "electrowinning":

At the anode: 4OH- → O2 + 2H2O +4e-
At the cathode: e- + [Au(CN)2]- → Au + 2CN-
Overall: 4OH- + 4[Au(CN)2]- → 4Au + 8CN- + O2 + 2H2O

The gold is "won" onto stainless steel electrodes or precipitated out as a fine black mud. The mud is then smelted and poured into moulds to make gold ingots.

As an laternative to this, zinc powder is added to the solution or reasonably pure of [Au(CN)2]-. This results in a metal displacement reaction:

2[Au(CN)2]-(aq) + Zn(s) -> [Zn(CN)4]2-(aq) + 2Au(s)

As before a fine black 'mud' of gold and residual zinc precipitates from the solution, which is then smelted.

Extraction of gold from seawater

There is some gold in seawater, but the concentration of dissolved gold is very low, perhaps 10 ng l-1. The most determined attempt to recover gold from sea water was undertaken by Fritz Haber, [F. Haber, Z. Angew. Chem. 1927, 40, 303.], who researched the matter extensively after the First World War. He wanted to find a way to pay Germany's war reparation debts. He developed a method involving gold reduction to the metal by sodium polysulfide and removal using sulphur-coated sand filters. Four expeditions were made on ships equipped with the extraction technology but with disappointing results. Following this Haber estimated the gold concentration in sea water to be 4 ng l-1, just one-thousandth of the amount which he had expected.

With the current estimate for the concentration of gold in sea water as 10 ng-1 and the total volume of the oceans at 1.37 x 109 km3, then the total quantity of gold dissolved in sea water is calculated to be 13.7 million tons. A lot of gold, but not extractable on a commercial basis as yet.



Discuss.